# Rare radiative charm decays within the standard model and beyond

## TL;DR

This paper provides Standard Model estimates for rare charm radiative decays, compares them with experimental data, and explores implications for new physics models like leptoquarks and SUSY, highlighting potential CP asymmetries and angular observables.

## Contribution

It offers the first comprehensive SM predictions for $c o u \, ext{gamma}$ processes and analyzes constraints on new physics from current measurements.

## Key findings

- Measured branching ratios exceed SM upper estimates.
- Constraints on dipole operators from $D^0$ decay data.
- Potential for large CP asymmetries in new physics scenarios.

## Abstract

We present standard model (SM) estimates for exclusive $c \to u \gamma$ processes in heavy quark and hybrid frameworks. Measured branching ratios ${\cal{B}}(D^0 \to (\phi, \bar K^{*0}) \gamma)$ are at or somewhat exceeding the upper range of the SM and suggest slow convergence of the $1/m_D, \alpha_s$-expansion. Model-independent constraints on $|\Delta C|=|\Delta U|=1$ dipole operators from ${\cal{B}}(D^0 \to \rho^0 \gamma)$ data are obtained. Predictions and implications for leptoquark models are worked out. While branching ratios are SM-like CP asymmetries $\lesssim 10 \%$ can be induced. In SUSY deviations from the SM can be even larger with CP asymmetries of $O(0.1)$. If $\Lambda_c$-baryons are produced polarized, such as at the $Z$-pole, an angular asymmetry in $\Lambda_c \to p \gamma$ decays can be studied that is sensitive to chirality-flipped contributions.